Rewrite async SelectMany to use pure Rx
Rewrite async SelectMany to use pure Rx

ReactiveUI patches in awaiters for Rx, we don't have them here until we
upgrade to Rx 2.2.2

bbqchickenrobot/Akavache,mms-/Akavache,PureWeen/Akavache,martijn00/Akavache,shana/Akavache,MathieuDSTP/MyAkavache,shana/Akavache,kmjonmastro/Akavache,christer155/Akavache,gimsum/Akavache,jcomtois/Akavache,Loke155/Akavache,MarcMagnin/Akavache,ghuntley/AkavacheSandpit,akavache/Akavache